myxovirus myx·o·vi·rus (mĭk'sə-vī'rəs)n. Formerly, any of a group of RNA-containing viruses with an affinity for mucins, now included in the families of Orthomyxoviridae and Paramyxoviridae. These viruses include the influenza virus, parainfluenza virus, respiratory syncytial virus, and measles virus.